Studies have been extended on the role of fibronectin in phagocytosis by monocytes and on phagocytosis by polymorphonuclear leukocytes and the HL60 myelomonocytic cell line. Although fibronectin causes phagocytosis of C3b coated erythrocytes by monocytes, it has no effect on phagocytosis by polymorphonuclear leukocytes. If PMN are first stimulated by C5a or f-met-leu-phe however, fibronectin has a dramatic effect on C3b mediated phagocytosis. HL60 cells will phagocytose C3b coated particles in the presence of fibronectin, if first exposed to DMSO, phorbolmyristic acetate, or lymphokine rich cell culture supernatant. The maximal effect of each of these agents appears after 3-5 days of co-culture with HL60. A mechanism for studying fibronectin binding to cell surfaces has also been developed. Fluoresced beads are coated with fibronectin, and their binding to cells studied with flow microfluorocytometry. Both monocytes and PMN as purified from perpheral blood will bind fibronectin coated beads; lymphocytes will not. HL60 cells do not bind these beads but do bind them after exposure to agents which induce differentiation. Monoclonal antibodies to the fibronectin molecule have been prepared. Of approximately 10 such monoclonals tested, two antibodies inhibit binding of Fn coated beads and also inhibit Fn mediated phagocytosis. Both monoclonals are IgG, and inhibit PMN and monocyte binding equally. The monoclonals will be used to identify and isolate the fibronectin fragment which binds to monocytes and PMN and to ascertain whether or not it is identical to the fibroblast cell binding fragment.